1. Field of the Invention
This invention relates to a device for reproducing a still picture and more particularly to the deflecting circuit of a reproducing device for obtaining a still picture by extracting a signal portion corresponding to a single field from the interlaced scanning television signal, by storing the extracted signal portion, and by repeatedly reading the stored signal.
2. Description of the Prior Art
Today, in the U.S., Japan etc., the exclusive television signal used in broadcasting is of interlaced scanning. In such a signal system, the period of one frame is chosen to be an odd multiple of a horizontal scanning period so that the horizontal sync signal may be received by the receiver every horizontal scanning period. Accordingly, in the television receiver adapted for the standardized television signal, the horizontal deflecting current in synchronism with the transmitted horizontal sync signal can be obtained by comparing the received horizontal sync signal with the horizontal flyback pulses produced by the horizontal deflecting circuit by means of a phase detector and by controlling the horizontal oscillator with the output of the phase detector. This means for obtaining the horizontal deflecting current is referred to as a horizontal automatic frequency control circuit (also referred to for brevity as horizontal AFC circuit).
Since the fluctuation of the horizontal repetition frequency of the television signal now used in broadcasting is rendered very small, the ac loop gain of the horizontal AFC circuit of the TV receiver can be chosen to be small. Accordingly, the noise band width can be made so small and therefore the S/N ratio can be improved.
Recently, a recording and reproducing device is widely used which displays a still picture by storing a signal portion corresponding to one scene of television signal and by iteratively reading the stored signal portion. In a device for recording and reproducing a still picture, a magnetic disc or the like which is rotated through 360.degree. for the period of scanning one frame or field, is used and the disc stores the television signal portion for one frame or field. In case where a signal portion corresponding to a single shot is extracted from the television signal representing a picture in motion and then stored, the signal portion most often covers a single field. If a signal portion corresponding to one frame of the television signal representing pictures in rapid motion, for which the television signal is different from field to field, is used to reproduce a still picture, then the reproduced picture will have double outlines which degrades the picture quality very. With the signal portion corresponding to one field, the vertical resolution will be degraded since the number of the scanning lines is decreased, but there is no double outline. Therefore, a field memory which stores a one-field signal (a signal portion corresponding to one field of the television signal) and from which the one-field signal is iteratively read out, is exclusively used in practice.
However, the horizontal AFC circuit used in an ordinary television receiver cannot be applied directly to a device for displaying the reproduced signal from the field memory. The reason is as follows. In the interlaced scanning system, the period of one frame is an odd multiple of one horizontal scanning period. The period of one field is, therefore, an integral multiple and a half of the horizontal scanning period. If a one-field signal is repeatedly read, there will be a difference of one half the horizontal scanning period, i.e. phase difference of 180.degree., between the starting points of the adjacent output signals. In order to reproduce a proper picture, the horizontal deflecting current must swiftly response to the phase shift of 180.degree. taking place every output. The HAFC circuit of an ordinary television receiver requires rather a long time (about 1/120 second, i.e. one half the period of a field) to cause the horizontal deflecting current to be synchronized with the sync signal under the existence of the phase shift. This means that about a half of the original picture is not properly reproduced, that is, the upper half of the reproduced picture flows horizontally.